Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical transmission systems. A wavelength division multiplexed optical transmission system employs plural optical channels, each channel being assigned a particular channel wavelength. In a WDM system, optical wavelengths are generated, each of which is modulated with a different data signal. The modulated wavelength may be referred to hereinafter as an optical signal. The modulated optical wavelengths (i.e., the optical signals) are multiplexed to form a WDM optical signal that can be transmitted over an optical fiber transmission path and demultiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers plural optical wavelengths can be directly amplified simultaneously, facilitating the use of wavelength division multiplexing in long-distance optical systems.
Optical add drop nodes located along the transmission path can add and remove data signals onto one or more wavelengths without requiring the termination of the other data signals on other wavelengths passing through the nodes—this is sometimes referred to as a transparent optical add drop node. In one transparent node, referred to as a reconfigurable optical add drop multiplexing (ROADM) node, multiple fibers in and out of the node can be supported and the different data signals can be selectively added or dropped or sent to different fibers based on the setting of an optical switch in the node.
For a so-called colored optical add drop node, the transceivers that generate the optical signals are connected to an add or drop port that only accepts data signals of a particular wavelength or set of wavelengths. Thus, the signal wavelength and band, although it may initially be tuned to match that fixed color input, becomes rigidly set in place once connected to the port. This approach has been widely used in the past, largely due to cost considerations. Recently, so-called colorless node techniques have been introduced. The optical ports for this type of node can accept any wavelength or set of wavelengths. Systems have been described in which a data signal that has been modulated onto one wavelength at its transceiver source is moved to a new wavelength in order to accommodate a new routing plan or network failure, for example. Changing the network configuration in this way, however, often has to be done very slowly in order to avoid disrupting data signals on other wavelengths. This is because the optical signals interact through the amplifiers and the nonlinear characteristics of the optical fiber. Furthermore, in normal operation the optical signals need to be repeatedly tuned and carefully adjusted in power as the properties of the fiber, amplifiers, and other components drift over time. When new optical signals are added to the network or any similar changes made, the adjustments likewise need to be done slowly and carefully. When a fiber break occurs the optical signals often experience power excursions that persist over long periods of time until the system controls can be adjusted to compensate and find the new operating points.